This invention relates to a recording/reproducing method of optical information, wherein a light flux emitted from a light source is converged on an information recording plane and optical information to be reproduced is recorded on an optical information recording medium, or information on the information recording plane is reproduced, to an optical pickup apparatus, to a converging optical system and an objective lens to be used therefor, and to a method of designing the objective lens.
In recent years, with the practical use of a red semiconductor laser of a short wavelength, development of a DVD (digital video disk or called also a digital versatile disk), which is a high-density optical information recording medium having a larger capacity but a size of the same degree as a CD (compact disk) which is a conventional optical information recording medium (also called an optical disk), has been promoted. In this DVD, the numerical aperture NA of the objective lens at the side of the optical disk is made to be 0.6 in the case where a short wavelength semiconductor laser of 635 nm is used. Further, a DVD has a track pitch of 0.74 xcexcm and a minimum pit length of 0.4 xcexcm, and is made to have a high density more than twice in comparison with a CD, which has a track pitch of 1.6 xcexcm and a minimum pit length of 0.83 xcexcm. Moreover, in addition to the above-mentioned CD and DVD, optical disks having various standards, for instance, such as a CD-R (a direct read after write, writing once compact disk), an LD, an MD (mini-disk), and an MO (magneto-optic disk) are merchandised and have come into general use. In Table 1, the thickness of the transparent substrate and the required numerical aperture of various optical disks are shown.
Besides, with regard to the CD-R, it is necessary for the light source to have a wavelength xcex=780 (nm), but for the other optical disks, a light source having a wavelength other than those noted in Table 1 can be used; in this case, the required numerical aperture NA should be varied in accordance with the wavelength xcex of the light source used. For example, in the case of a CD, the required numerical aperture is approximated by NA=xcex (xcexcm)/1.73, and in the case of a DVD, it is approximated by NA=xcex (xcexcm)/1.06.
Further, the numerical aperture as referred to in this specification (for example, referred to as NA1, NA2, NAL, NAH, NA3, NA4, etc. hereinafter) means the numerical aperture of the converging optical system as seen from the transparent substrate side. NA1 is a numeral aperture necessary for reproducing information from or recording information in the first optical information recording medium and NA1 is a numeral aperture necessary for reproducing information from or recording information in the second optical information recording medium.
As described in the above, it is now an age when various kinds of optical disks, which are different in the size, the thickness of the transparent substrate, the recording density, the wavelength used, and so forth, are available on the market, and optical pickup apparatuss capable of being used for various kinds of optical disks are proposed.
As one of them, it has been proposed an optical pickup apparatus which is equipped with converging optical systems corresponding to the different optical disks respectively, and the converging optical systems are switched over in accordance with the optical disk to be reproduced. However, in this optical pickup apparatus, a plurality of converging optical systems is required and it brings not only a high cost but also the necessity of a driving mechanism for switching over the converging optical systems; accordingly, the device is not desirable because of the complexity and the requirement of the precision in switching over.
Therefore, various kinds of optical pickup apparatuss which can reproduce a plurality of optical disks by using a single converging optical system.
Incidentally, in this specification, the phrase xe2x80x98to reproduce an optical diskxe2x80x99, xe2x80x98to record an optical diskxe2x80x99 or the like is used in the sense xe2x80x98to reproduce information in an optical disk, xe2x80x98to record information in an optical diskxe2x80x99, or the like for simplicity""s sake.
As one of them, in Japanese laid open patent H7-302437, an optical pickup apparatus, wherein the refracting surface of the objective lens is divided into a plurality of ring-shaped domain, and each of the divisional surface domains makes the beam converge on one of the optical disks having different thickness to reproduce information, is described.
Besides, in Japanese laid open patent H7-57271, an optical pickup apparatus, wherein, in the case of the first optical disk with the thickness of the transparent substrate t1, an objective lens designed to make the wave front aberration included in the converged beam not larger than 0.07xcex is employed, and in the case of the second optical disk having the thickness of the transparent substrate t2, the converged beam spot is formed in a little defocused state, is described.
However, in the optical pickup apparatus described in Japanese laid open patent H7-302437, it is necessary to make the laser output large because the incident light quantity is simultaneously divided into two focal points by a single objective lens, which brings about high cost. Further, in the optical pickup apparatus described in Japanese laid open patent H7-57271, increasing of jitter owing to side lobe occurs when the reproduction of the second disk is carried out. In particular, because the second disk is subjected to reproduction forcibly by the objective lens which makes the wave front aberration not larger than 0.07xcex for the first optical disk, there is a limit in the numerical aperture capable of reproducing the second optical disk.
Further, various kinds of optical pickups being equipped with a plurality of laser light sources corresponding to the wavelength to be used, wherein the laser beam is converged on a recording plane with a necessary numerical aperture by the same objective lens, have been proposed (for example, Japanese laid open patents H8-55363, H10-92010, etc.). However, because of this, the structure of the optical system becomes complex, it has raised a problem such as that the number of parts used is increased with it or that the performance is lowered.
The inventors have developed a special objective lens composed of a plurality of ring-shaped zones which are formed by dividing the whole surface into concentric circles, wherein each of the ring-shaped zones is corrected for the aberration within the diffraction limit with regard to each pertinent optical information recording medium by positively utilizing the spherical aberration produced owing to a plurality of light sources having different wavelength and/or the transparent substrates with different thickness of recording surface, and have previously proposed an optical pickup with a simplified structure using this objective lens (Japanese patent application H9-286954).
This objective lens has a function such that a necessary numerical aperture can be automatically obtained in accordance with the wavelength used and/or the thickness of the transparent substrate. However, there is a problem that the light flux from a ring-shaped zone corresponding to a different optical information recording medium has its light spot intensity decreased by a phase shift, and can not be utilized in recording and reproducing.
The above-mentioned problem can be solved by the following structure.
The converging optical system for an optical pickup of this invention is composed of two laser light sources with different wavelengths, a converging optical system including an objective lens for converging the laser beams from the light sources, and a photo-sensing element for detecting the light reflected from the recording plane, and is an optical system for an optical pickup for use in recording and reproducing at least two kinds of optical information recording media with different thickness of the transparent substrate,
said optical system further comprising means for dividing the light flux of the converging optical system into three light flux on the ring-shaped zones from the neighborhood of the optical axis to farther position, said light flux being successively the first, second, and the third light flux from the neighborhood of the optical axis to farther position, wherein the first light flux is used for all of the recording media with the different thicknesses of the transparent substrate, the second light flux is used mainly for the recording media with a thick transparent substrate, and the third light flux is mainly for the recording media with a thin transparent substrate,
said optical system being characterized by it that at least one of said three divisional light flux has its phase shifted against other light flux in order that the light spot intensity may be raised for the two recording media which are different in the wavelength and the thickness.